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APPLICATION NOTE

High-Resolution 3D Confocal
Raman Imaging for Group III
NitridesConfocal Raman Microscopy
The outstanding sensitivity of WITec‘s optical systems reduces acquisiton times for single Raman spectra down
to well below 1 ms and enables ULTRAFAST RAMAN IMAGING® and time-resolved micro-Raman spectroscopy
with unprecedented resolution. Detecting signals from weak Raman scatterers or extremely low material concentrations or volumes with the lowest excitation energy levels is the unrivaled advantage of WITec sytems. All
Raman spectra are collected and processed by the WITec software suites, which also provide for post-processing
and in-depth analysis of spectral data.

High-resolution 3D Confocal Raman Imaging for Group III Nitrides
or serve for embedded periodic deflectors
to enhance light extraction [1]. Figure 1
shows a cross-sectional SEM micrographof the sample. It consists of the patterned
sapphire with troughs and ridges at thebottom followed by GaN grown in thetroughs and in part of the ridge edges.

Introduction
Structured substrates are widely employed
in semiconductor research and especiallyin current semiconductor development.
Demanding requirements in device quality
and reliability make it increasingly important to have a detailed knowledge of theinherent strain and crystalline propertiesof device structures. X-ray diffraction iscommonly used in order to probe film
thicknesses, lattice constants and strainstates of layer structures and scanningelectron microscopy (SEM) is used to inspect surfaces and defects in the structure
to understand the growth history. In thisapplication note results are presented of3D confocal Raman imaging measurements that reveal changes in the signal
which can be attributed to strain as well as
to changes in the lattice structure.

Sample description
The sample used was an unintentionallydoped GaN layer of 20 μm thickness grownby metal organic vapor phase epitaxy(MOVPE) and subsequent overgrowth by
hydride vapor phase epitaxy (HVPE) on aa)

Fig. 1: SEM image of the structures. Vertical lines
are steps due to cleavage with a higher line
density in regions of direct contact between GaNand sapphire.

c-plane patterned sapphire substrate (PSS).
Fabrication of the pattern and growth
was performed at the Ferdinand-Braun
Institute in Berlin. The sapphire substrate
was patterned with I-line beam stepper
lithography using a KMPR 1000 negative
photoresist and ion coupled plasma (ICP)
etching. The investigated honeycomb-likestructure has a trough depth of 4.5 μm
with a ridge width of 3.5 μm and a pitch of
12 μm.

Finally a coalesced GaN layer covers thepattern with slight contrast above thetrough and ridge regions. Vertical lines arecleaving steps which are denser in the GaN
directly grown on the sapphire surface inthe troughs or on top of the ridges. Figure
2a, b, and c contain SEM images of thepattern after the ICP process (a), after thegrowth of the first 3 μm thick GaN film on
the PSS substrate by MOVPE (b), and after
the growth of another 17 μm thick GaN
layer by HVPE (c).

This and similar patterns are relevant
for light emitting devices as they allow
for epitaxial lateral overgrowth (ELO) toreduce threading dislocation density andimprove the internal quantum efficiencyb)c)

Fig. 2: SEM Images of the structures after the etching process (a), after the growth of the first 3 μm GaN film (b), and after the growth of the additional 17 μm thick GaN
layer (c).

Experimental section
The samples were examined using analpha300 confocal Raman microscopewith a frequency-doubled Nd:YAG laser
for excitation (λ=532 nm). The light was
coupled into the microscope using a photonic fiber and brought onto the sample
using a dichroic mirror and a 100x NA 0.9
air objective. In the detection beam path,
the Rayleigh-scattered light was filteredout using an edge filter and a 50 μm
core-diameter fiber was used as a confocal
pinhole, providing good depth resolution
while maintaining excellent collection
efficiency. The light was then directed to a
UHTS300 spectrometer equipped with an
1800 g/mm grating (BLZ 500 nm) and an
EMCCD camera. The XY positioning of thesample was achieved using a piezoelectricscanning stage and a stepper motor wasused for focus control (10 nm per singlestep).

Results
The Raman spectra were recorded as adepth-scan along an interesting axis along
the sample (240 x 80 points on a scan areaof 60 x 20 μm2) as shown in figures 3 and4. 3D stack scans were additionally performed and analyzed (180 x 45 x 20 points
with a scan volume of 60 x 15 x 20 μm3).
For each of the scans a spectrum wasrecorded at every pixel with an integration time of 20 ms/spectrum. Using theWITec Suite FIVE software, the relevant
information was extracted and displayed
as color-coded Raman images. The results
enabled the generation of a 3D image of
the Raman features throughout the investigated sample volume.

Fig. 3: Stitched image of the sample acquired using a 100x objective. The
red square shows the position of the high-resolution video image shownin Figure 4.

Figure 3 shows the white-light image ofthe sample acquired using the built-in
ocular video camera and the stitching
function of the system. The image consistsof 100 individual images and allows for alarger overview with high-resolution. The
red square in the image indicates the areafrom which the single high-resolution
white-light image using the 100x objective
was taken (Figure 4). In this image theholes in the structure appear in different
colors due to differences in the whitelight interference, which depends on thedistance between the upper surface of
the GaN grown in the holes and the lower
GaN surface of the coalesced layer. Thegreen area in figure 4 shows the area atwhich the XY scan and the stack scan wasperformed and the red line indicates theplane in which the single depth-scan wasperformed.

Fig. 4: Video image of the area of interest. The area of the stack scans isshown in green and the red line shows the position of the single depthscan.

Analysis of various spectral features in thesample
Using a depth-scan, three areas could bedetected in the sample. Figure 5 shows thefalse-color Raman image with the spectra
in corresponding colors shown in Figure 6.
In the magenta areas, the spectrum shows
the typical features of GaN spectra [2]. In
the green areas, which coincide with theedges of the ridges, an enhanced fluorescence can be seen. This fluorescence islocated primarily in the low-wavenumber range of the spectrum (see green
spectrum in Figure 6). The red areas arelocated directly above the holes and canbe spectrally characterized by a strong
reduction of the intensity of the peak near
735 rel. cm-1 (relative wavenumbers) anda broad peak on the higher wavenumber
side of this peak. This is shown in thezoomed view of the relevant spectral detail(Figure 6). This peak shift and broadening
may hint at a material of lower crystalline
quality. The
inclusion of contaminations in non-c-planar growth at these positions could giverise to the coupled non-planar opticalphonons A1(LO)+ or A1(L)+.
Figure 7 shows one layer extracted fromthe stack scan in which the typical spectralfeatures discussed in the last paragraph
are again displayed using the same colorcode. The fluorescence (green areas) islocated at the walls of the ridges and theareas of lower intensity of the 735 rel. cm-1
peak (shown in red) are located on theinteriors of the holes.
The layers recorded in the image stack
were evaluated in a manner similar tothe layer shown in Figure 7 and combined
into a 3D reconstruction using the ImageJsoftware. Figure 8 shows a 3D view of thereconstructed 3D structure. As before inFigure 7, the location of the fluorescenceand the regions of a weaker 735 rel. cm-1
peak can be clearly identified at the positions of the holes.

Fig. 5: Color-coded Raman image of the depth scan performed along the red line shown in Figure 4. The magenta,
red and green areas correspond to the spectra shown in Figure 6. The holes in the structure were located precisely
underneath the red areas. The height of the structure appears to be reduced due to the difference in the refractive
index between air (~1) and GaN (2.45).
Fig. 6: Color-coded Raman
spectra corresponding tothe Raman image shownin Figure 5. In the zoomed
view the green spectrum isomitted for clarity.

Fig. 7: Color coded Raman image of a single plane extracted from the stack scan. The magenta, red and green
domains correspond to the spectra shown in Figure 6. The honeycomb-like holes in the structure were locatedprecisely beneath the red areas and the yellow color is a combination of the red and the green.
Fig. 8: Color-coded
3D Raman Image ofthe structure. Thecolors correspondagain to thecolors of the spectra
shown in Figure 6.

Peak-shift analysisIt has been shown by various authors[3,4,5] that strain in crystalline samples
can be correlated with Raman peak shift.
Using the measurements presented above,a peak-shift analysis of the spectra wasperformed. For this purpose the peak near
570 cm-1 relative wavenumbers of each
spectrum in each of the scans was fittedusing a Lorentzian curve. This results
in a very accurate determination of theposition of the Raman peaks with a typicaluncertainty of 0.02 rel. cm-1.
Figure 9 shows the position of this peakfor the depth-scan. There are various
areas in which the Raman line is shiftedto lower wavenumbers (green) as well as
various areas where it is shifted to higher
wavenumbers (red). Two representative
spectra illustrating these peak shifts areshown in Figure 10. Here experimental
data recorded by the CCD camera as wellas the Lorentzian fitting curve are shown.

Figure 11 shows the distribution within thesample in one of the planes extracted fromthe stack scan. Again various domains canbe seen. However, these discrete images
in single planes make it quite difficult todraw a clear picture of the peak shift andthe resulting strain distribution in thesample. Using the complete informationcollected by the stack scan and performing a 3D reconstruction permits a muchclearer analysis.
Figure 12 shows this 3D distribution
with shifts to lower wavenumbers again
marked in green and shifts to higher
wavenumbers shown in red. The image
underneath is the intensity of the peaknear 735 rel. cm-1 of a plane just above theholes and allows for the spatial associationof the stress fields with the position of theholes. It is readily apparent that the stressfields propagate from the interface tothe surface mainly in tube-like structures,
which was not obvious in the 2D views ofthe sample.

The overall differences in the positionof the peak are quite small (< 1 cm-1) andthus the overall differences in the strain of
the GaN are also small. The Raman stressfactor for the E2high phonon mode, however,is quite high with –3.4 +-0.3 cm-1/GPa [6].
Minute changes in the lattice constantcan be measured due to the high Raman
stress factor making the peak shift visible
within highly sensitive Raman spectroscopy. [7]. Surprisingly, the compressive strain
in this GaN sample is very homogeneous
independent of the region (hole or ridge)
with the exception of a small, less compressively strained border at the ridgeedges where some disturbed GaN growthat the edge walls was observed. Therefore,
Raman microscopy has been shown to be
an extremely sensitive and useful tool for
growth optimization on patterned sapphire substrates.

Fig. 9: Color-coded peak-shift image of the structure
as measured using a depth-scan. The shift of the main
peak is shown in this image.

Fig. 10: Spectra illustrating the peak shifts. The data
points as well as the fitted curve are displayed in thecolors corresponding to Figure 9.

Fig. 11: Color-coded Peak-shift image of the structure
extracted from one layer of the stack scan. The shift ofthe main peak is shown in this image.

Fig. 12: 3D View of the stress fields. The image in thebottom plane shows the intensity of the peak near 735
rel. cm-1 in an orientation indicating where the holes inthe structure are located.

Conclusion

Acknowledgement

References

3D confocal Raman imaging was used toprobe the properties of GaN layers grownon patterned sapphire substrates. It wasshown that an enhanced fluorescencesignal can be detected along the perimeter of the etched structures and that thematerial above this region shows a clearlymodified Raman spectrum.

WITec would like to thank Dr. Eberhard
Richter from the Materials Technology
Department of the Ferdinand Braun Institute, Berlin, Germany (www.fbh-berlin.de)for providing the sample along with theinvaluable scientific discussion leading tothis application note.

[1] M. Cooke, Semiconductor Today, Compounds & Advanced Silicon “Technology focus: Nitride substrates”
Vol. 6 (7) Sept./Oct. 2011 p. 120.

A stress analysis using the exact positionof the Raman E22 phonon mode peaks
showed tube-like structures where theRaman peaks were down-shifted (lesscompressive), which propagate from theGaN-sapphire interface up to the surfaceof the GaN layer. This structure is in good
agreement with partially distorted and unintended growth at the edge walls of theridges observed in the SEM image. While
SEM allows for the examination of physicalsurfaces, Raman has the advantage ofbeing non-destructive and providing very
detailed insight into the variations of thestrain in group III nitride structures grownon patterned sapphire substrates. The twotechniques therefore complement eachother ideally.

[2] M. Kuball, “Raman spectroscopy of GaN, AlGaN,
and AlN for process and growth monitoring/control”,
Surf. Interface Anal. 31 (2001) 987–999.
[3] P. Gundel, M.C. Schubert and W. Warta, “Simultaneous stress and defect luminescence study on silicon”,
Phys. Status Solidi A 207 (2010) 436-441.
[4] A.M. Gigler, A.J. Huber, M. Bauer, A. Ziegler, R. Hillenbrand and R.W. Stark, „Nanoscale residual stress-field
mapping around nanoindents in SiC by IR s-SNOMand confocal Raman microscopy”, Optics Express 17(2009) 22351-22357.
[5] T. Wermelinger, C. Borgia, C. Solenthaler and R.
Spolenak, “3-D Raman spectroscopy measurementsof the symmetry of residual stress fields in plastically
deformed sapphire crystals”, Acta Materialia 55 (2007)
4657-4665.
[6] I. Ahmad, M. Holtz, N.N. Faleev, H. Temkin,“Dependence of the stress–temperature coefficienton dislocation density in epitaxial GaN grown on aAl2O3 and 6H–SiC substrates“, J. Appl. Phys. 95 (2004)
1692-1697[7] I. Gorczyca, N.E. Christensen, E.L. Peltzer y Biancá,
C.O. Rodriguez, “Optical phonon modes in GaN andAlN”, Phys. Rev. B 51 (1995) 11936-11939

WITec alpha300 Confocal Raman Microscope Seriesalpha300 series: modular and flexible design guarantees advanced confocal Raman imaging withmultiple correlative microscopy technique options, such as AFM, SNOM, SEM (RISE), fluorescence,photoluminescence and topographic Raman imaging (TrueSurface).